Acoustic well logging comprises the measure of various acoustic properties of formation penetrated by a well borehole. These measured properties are subsequently used to determine formation and borehole properties of interest including, but not limited to, formation porosity, formation density, stress distribution, formation fracturing, and formation anisotropy.
Elastic anisotropy manifests itself as the directional dependence of sound speed in earth formation. Anisotropy in earth formation may be due to intrinsic microstructure such as the case in shales, or may be due to mesostructure such as fractures, or may be due to macrostructure such as layering due to sedimentation. Whatever the cause for anisotropy may be, good estimates of elastic properties of anisotropic media are required in resolving seismic images accurately, in interpreting borehole logs and in estimating drilling mechanics parameters. Specifically, seismic lateral positioning, amplitude versus variation with offset (AVO) and vertical seismic profiling (VSP) interpretation, borehole stability and closure stress estimates are all dependent on an accurate and precise measure of the degree and configuration of anisotropy of subsurface formations. Even though prior art acoustic logging systems, such as the crossed dipole systems, have advanced the state of the art, the present state of borehole acoustic logging has not been able to measure the structure of elastic anisotropy in a consistent manner.
Prior art multimode acoustic logging systems are typified by acoustic transmitter excitation with radiation patterns of 2nth order poles such as monopole (n=0), dipole (n=1) and quadrupole (n=2) as well as reception by a plurality of receivers with similar discrimination patterns. Examples of prior art systems are disclosed in Aron et al, “Real-Time Sonic Logging While Drilling in Hard and Soft Rocks”, Paper HH, SPWLA 38th Annual Logging Symposium, 1997; Tang et al, Chapter 5, “Quantitative Borehole Acoustic Methods”, Elsevier, 2004; Varsamis et al, “LWD Shear Velocity Logging in Slow Formations Design Considerations and Case Histories”, SPWLA 41st Annual Logging Symposium, 2000; U.S. Pat. No. 5,753,812 “Transducer for Sonic Logging While Drilling”, Aron, J., et al; and U.S. Pat. No. 6,213,250 “Transducer for Acoustic Logging, Wiesniewski, L. et al. Accordingly, a monopole measurement system will typically comprise one or more monopole transmitters and monopole receivers, and a dipole measurement system will typically comprise one or more dipole transmitters and dipole receivers. Prior art indicates, however, that when the axis of a borehole logging tool axis is eccentered or tilted with respect to the borehole axis, other undesired modes are also created in the borehole/formation system thereby contaminating the measurements of interest (see Leslie et al, “Eccentric Dipole Sources in Fluid-Filled boreholes: Numerical and Experimental Results”, Journal of the Acoustical Society of America, Vol. 87, No., 6, pp. 2405-2421, 1990). The ability of the receivers to discriminate and filter out the unwanted modes is compromised by azimuthal aliasing as well as any phase and sensitivity mismatch of the plurality of receivers.
Prior art multipole acoustic logging systems comprise separate transducer systems for excitation and detection of each order multipole. Such a system is disclosed by Pistre et al., “A Modular Wireline Sonic Tool for Measurements of 3D (Azimuthal, Radial, and Axial) Formation Acoustic Properties”, SPWLA 46th Annual Logging Symposium, Jun. 26-29, 2005.
There is no known acoustic logging system that can efficiently generate and sense multiple modes simultaneously without unacceptable degradation in performance due to eccentering, tilt or receiver phase mismatch.
The prior art directed toward multipole mode acoustic logging systems is typically complex and is often impractical in a real-time borehole logging system. In addition, inaccuracies inherent in practical borehole measurements and the accuracy of required unknowns, such as formation elastic moduli or stress conditions, do not warrant such system complexity. As an example, a method commonly used in determining acoustic wave velocities (or slownesses), namely, the semblance technique, is more influenced by the group velocity rather than the phase velocity. In anisotropic media, the group and phase velocity vectors do not necessarily coincide thereby leading to measurement inaccuracies of indeterminate order. A prior art method for evaluating elastic wave velocities in anisotropic formations from borehole logging is disclosed in U.S. Pat. No. 6,772,067. This is a typical method employed in wireline acoustic crossed dipole logging. However, the performance of method is sensitive to the effects of eccentricity, tilt and receiver mismatch.